Configuring EIGRP Split Horizon

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Real World Application & Core Knowledge

In the previous lab you configured static neighbors on a frame relay hub-and-spoke between R1 and R2, R1 and R3, R1 and R4. After verification you should have noticed that routes from R2 did not get propagated to R4 through the hub. This is by default the normal operation of EIGRP and this is caused by a loop prevention mechanism called split-horizon.

The split horizon rule simply states that routes will not be advertised back out an interface in which they were received on. After all; if a router sends route updates to a neighbor why would that router need to have the neighboring router re-advertise those routes back to the originating router? The simple answer is that its not needed.

However in some scenarios EIGRP IP split horizon is required to be disabled to ensure intended operation; for example a hub and spoke topology where the physical interface has multiple IP’s mapped to specific PVC’s out a single physical interface. In this case the normal behavior is that routes learned via an interface will not be re-advertised back out that interface so in that case, with the previous lab R2’s routes being advertised to R1’s Serial0/0 interface would not be re-advertised back out R1’s Serial0/0 interface to R3 and R4.

In a case like this you’d need to disable EIGRP split-horizon using the no ip split-horizon eigrp as# command in interface configuration mode. This disables split-horizon on a per interface basis for the specified EIGRP autonomous system.

This lab will continue to build upon the topology used in Lab 8-2 and other labs that are found in Section 8.

Familiarize yourself with the following new command(s);

Command

Description

no ip split-horizon eigrp as#

This command is executed in interface configuration mode to disable ip split-horizon for the specified EIGRP autonomous system.

Lab Prerequisites

If you just completed Lab 8-2 you may start where you left off, if not you can load the Free CCNA Workbook GNS3 topology; start and establish a console session with R1, R2, R3, R4 and R5 then load their initial configurations included below by copying the config from the textbox and pasting it into the routers console.

As shown above you can see that R5 now has routes to R2’s Lo0 interface and the next hop to that destination is R4 so with that in mind, R4 also knows how to get there otherwise it would not advertise that specific route.

To verify that R5 has ip reachability to R2’s Lo0 interface you can pink R2’s Lo0 interface from R5’s Lo0 interface as shown below;